Production of oxide coatings on ferrous surfaces and mechanically working the same



United States Patent O Fritz Singer, Starnberg, Upper Bavaria, Germany Application November 5, 1951, Serial No. 254,983

Claims priority, application Germany December 23, 1948 11 Claims. (Cl. 148-635) No Drawing.

This invention relates to and has for its object the processing of iron and steel and articles having a ferrous surface to produce thereon uniform, strongly adherent coatings.

The invention further relates to and has for its objects the process of treating iron and steel and articles having a ferrous surface to form a strongly adherent coating thereon to facilitate the plastic deformation of the product so coated. More particularly, the invention contemplates a process of facilitating the plastic deformation of iron and steel and articles having a ferrous surface by coating the same with a uniform, strongly adherent coating consisting mainly of hydrated ferric oxide, and then subjecting the coated metal to plastic deformation.

A further object of the invention resides in a novel process of forming a uniform, strongly adherent coating consisting mainly of hydrated ferric oxide on iron and steel and articles having a ferrous surface, by steps which comprehend the formation of a film on the same by applying thereto a dilute aqueous solution of an acid or an acid-reacting salt, and then without washing or removing the said film, converting the same to a coating consisting mainly of hydrated ferric oxide; and more specifically, subjecting the said film to the action of air in the presence of water vapor to form the said coating consisting mainly of hydrated ferric oxide.

Other objects of the invention will be apparent from the following description and appended claims.

In my United States Patent No. 2,105,015, 1 have described a process for facilitating the plastic deformation of iron or steel consisting essentially in forming a phosphate or other crystalline coating on the article to be deformed. This process is now extensively used for facilitating the cold drawing of wire and tubes.

According to the present invention, a uniform, strongly adherent coating consisting mainly of hydrated ferric oxide is produced on a ferrous surface by forming on the surface a film of a dilute aqueous solution of acid or acid-reacting salt and then without washing the surface subjecting it to the action of air in the presence of water vapor. i find that such a coating also facilitates the plastic deformation of the article coated with it. In cold-drawing and similar processes a thin coating is better than a thick one, but the thinner the coating, the more frequently it is necessary to renew it if the maximum reduction in cross-section is to be obtained. The process according to the invention presents the advantage that the coating can be applied cheaply without the use of either any special apparatus or heat, and therefore can be renewed frequently without substantially increasing the cost of the cold-drawing or like process.

In the drawing of wire or tubes it is known to pickle the wire or tube in fairly strong acid, remove it from the acid and let it remain in air overnight so that rust forms, and it is also known that this rust facilitates the subsequent drawing. However, in these pickling processes the wire or tube is always washed after being removed from the pickling bath, and the rust coating is irregular, being r, 2,728,696 Patented Dec. 27, 1955 caused by traces of acid still adhering to parts of the ferrous surface. In the process according to the present invention the surface is not washed, but rather a uniform thin film of the solution of acid or acid salt is left on it. This film will dry in a few minutes, and it may be allowed to dry before the article is subjected to the action of air and water vapor, or the article may be forthwith subjected to this action while still wet. In any case the film insures the production of a uniform oxide coating which is firmly bound to the surface. The action of the acid or acid salt appears to be essentially catalytic.

It is most convenient to form the coating by dipping the article in the solution and, after taking it out again, removing the bulk of the adherent solution by draining or shaking, primarily in order to recover this for further use. The solution may alternatively be applied by brushing or spraying. In any case the surface should be free from scale when it is dipped in or otherwise comes into contact with the solution, so any adherent scale must first be removed mechanically or by pickling.

The coating formation begins with the drying of the acid film, which produces a grey color on the ferrous surface, no doubt because the ferrous salt of the acid is formed. After a short time in the air, the color changes to green, which gradually deepens to very dark green. Probably this dark green film or coating consists of hydrated ferroso-ferric oxide. Continued action of the air causes the dark green color to change to a shiny redbrown, which is characteristic of the final coating and shows that this contains ferric oxide.

It is essential that there should be enough moisture for these reactions to proceed. if the air is nearly dry the reaction will often not proceed beyond the formation of a ferrous salt, particularly with sulphuric acid. Preferably the relative humidity on moisture content is between 60 and The acid or acid-reacting salt used mustbe of such a nature as to etch the metal surface, that is to say it must form some salt with the metal of the surface. In carrying out the invention to produce the desired oxide coating quickly, the acid concentration of the solution is of importance. if this is too high, the oxide coating formation takes place slowly and in addition the oxide coating is not as uniform as it should be. The preferred acid is hydrochloric acid, and with it the acid concentration of the solution should not be greater than 3% nor less than 0.2%. Other acids may be used in equivalent amounts.

With the use of a 1.5% solution of hydrochloric acid, the oxidation process takes about 1 hour at ordinary atmospheric temperature and with air having a relative moisture content of say 60%. If the acid concentration exceeds 3%, the dark green film or coating is only partly converted into the red-brown coating after one hour. If the formation of the hydrated iron oxide coating is interrupted at this stage, the film or coating is found not to be firmly bound to the surface, but can be removed from it by brushing in a stream of water.

Hydrochloric acid is preferred because it is more effective than other acids, no doubt because of the greater mobility of the chlorine ions. Sulphurous acid also gives excellent results. Sulphuric and phosphoric acids can be used, provided that in the subsequent oxidation the moisture content of the air is very high. Even so, there is a tendency for the oxidation reaction to stop at the stage at which ferrous sulphate or ferrous phosphate is formed and the addition of such an agent as sulphurous acid to the oxidizing atmosphere may be necessary to produce the desired coating.

Instead of using an acid a dilute solution of an acid salt, e. g. a hydrogen salt or the salt of a weak base and a strong acid, which partially dissociates in an aqueone solution to yield an acid may be used. Examples of such salts are ferric chloride and chromium chloride.

The hydrated ferric oxide coatings produced according to the invention are primarily intended to replace phosphate coatings used in the plastic deformation of iron and steel and accordingly they should present the same characteristics as phosphate coatings in accordance with the particular purpose for which they are acquired. In particular the thickness, smoothness, degree of adhesion and capacity for absorbing oil should all be made the same as those of the corresponding phosphate coat- As in the production of phosphate coatings, the formation of a coating of hydrated ferric.oxide can be accelerated by the use of an accelerator, which may be of the same type as that used in accelerating the formation of a phosphate coating on a ferrous surface. Sulphites and thiosulphates which precipitate colloidal sulphur when in acid solution in contact with iron are particularly effective accelerators. Formic acid may also be used. In addition, however, the production of the hydrated ferric oxide coating can be accelerated by materials such as glycerine and calcium chloride which are hygroscopic and so maintain moisture on the ferrous surface.

In carrying out the process the articles after being removed from the solution of dilute acid or acid salt may be introduced into closed chambers equipped with air-conditioning apparatus so that a humid atmosphere is maintained. The speed of the oxidation reaction depends on the temperature and moisture content of the air, but the thickness of the film depends not only on these factors but also on the duration of the treatment.

As an example, ferrous parts which have been descaled either mechanically or chemically may be dipped into a solution consisting by volume of parts of hydrochloric acid (20 B.), 90 parts of water and 1 part of 6% sulphurous acid, this solution being at a temperature of from to C. The amount of acid which remains adherent to the ferrous surfaces when they are removed from the solution depends upon the quality of the surface, rough surfaces holding more acid than smooth ones. Accordingly the duration of the dipping may be varied, being from a few seconds with a rough surface, to from 5 to l0 minutes with smooth surfaces. When the parts have been removed from the acid solution they are allowed to drain and then introduced into a chamber in which the relative moisture content is more than 60%. If no apparatus for maintaining the moisture content constant is available a pan filled with water to a temperature from 20 to C. should be put in the bottom of the chamber. Under such conditions a hydrated ferric oxide coating will be formed uniformly over the whole ferrous surface in from 1 to 1 /2 hours. If the desired moisture content of the atmosphere cannot be maintained, from 2 to 5 parts of glycerine may be introduced into the acid solution in which the ferrous parts are dipped.

An example of another suitable solution is one containing (W./v.) 6 parts of sodium sulphite crystals, 4 parts of hydrochloric acid (20 B.) and 90 parts of water. Yet another example consists of 5 parts of chromic chloride hexahydrate crystals in 95 parts of water. The chromic chloride has such hygroscopic properties that there is no need in this case to use an atmosphere of high relative moisture content.

The hydrated ferric oxide coating produced according to the invention may also be used to bond paint to iron or steel. However, it is not stable if it is subjected to air supersaturated in water vapor for any length of time,

so it is desirable to paint the iron or steel immediately after the formation of the coating.

The hydrated ferric oxide coatings produced according to the invention can be converted into black coatings of iron sulphide by treatment with dilute alkali sulphides in the cold. The resultant sulphide coatings are superior 4 to either phosphate or hydrate ferric oxide coatings in facilitating the cold-working of iron and steel.

I claim:

1. The process for producing a uniform, smooth, bright, and strongly adherent coating consisting mainly of hydrated ferric oxide on the surface of iron and steel articles, and other articles having a ferrous surface, which comprises providing said surface with a film of an aqueous solution of a chemical of the class consisting of acids and acid-reacting salts, said solution having a free acid content equivalent to about 0.2% to 3% hydrochloric acid, drying said film on said surface, and subjecting the filmed surface in dry form to the action of air having a relative humidity such as to cause the reaction to go to the ferric oxide stage, until the surface is uniformly covered with a red-brown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface.

2. The process for producing a uniform, smooth, bright, and strongly adherent coating consisting mainly of hydrated ferric oxide on the surface of iron and steel articles, and other articles having a ferrous surface, which comprises providing said surface with a film of an aqueous solution of a chemical of the class consisting of acids and acid-reacting salts, said solution having a free acid content equivalent to about 0.2% to 3% hydrochloric acid, drying said film on said surface, and subjecting the filmed surface in dry form to the action of a controlled atmosphere of air having a relative humidity in excess of about until the surface is uniformly covered with a redbrown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface.

3. The process for producing a uniform, smooth, bright, and strongly adherent coating consisting mainly of hydrated ferric oxide on the surface of iron and steel articles, and other articles having a ferrous surface, which comprises providing said surface with a film of an aqueous solution of a chemical of the class consisting of acids and acid-reacting salts, said solution having a free acid content equivalent to about 0.2% to 3% hydrochloric acid, drying said film on said surface, and subjecting the filmed surface in dry form to the action of a controlled atmosphere of air having a relative humidity of about 60% to until the surface is uniformly covered with a red-brown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface.

4. The process for producing a uniform, smooth, bright, and strongly adherent coating consisting mainly of hydrated ferric oxide on the surface of iron and steel articles, and other articles having a ferrous surface, which comprises providing said surface with a film of an aqueous solution of a chemical of the class consisting of acids and acid-reacting salts, said solution having a free acid con centration of about 1.5% to 3%, drying said film on said surface, and subjecting the filmed surface in dry form to the action of a controlled atmosphere of air having a relative humidity of about 60% to 80%, until the surface is uniformly covered with a red-brown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface.

5. The process for producing a uniform, smooth, bright, and strongly adherent coating consisting mainly of hydrated ferric oxide on the surface of iron and steel articles, and other articles having a ferrous surface, which comprises providing said surface with a film of an aqueous solution of an inorganic acid having a free acid content equivalent to about 0.2% to 3% hydrochloric acid, drying said film on said surface, and subjecting the filmed surface in dry form to the action of air having a relative humidity such as to cause the reaction to go to the ferric oxide stage, until the surface is uniformly covered with a redbrown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface.

6. The process for producing a uniform, smooth, bright, and strongly adherent coating consisting mainly of hydrated ferric oxide on the surface of iron and steel articles, and other articles having a ferrous surface, which comprises providing said surface with a film of an aqueous solution of sulfurous acid having a free acid content equivalent to about 0.2% to 3% hydrochloric acid, drying said film on said surface, and subjecting the filmed surface in dry form to the action of air having a relative humidity such as to cause the reaction to go to the ferric oxide stage, until the surface is uniformly covered with a red-brown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface.

7. The process for producing a uniform, smooth, bright, and strongly adherent coating consisting mainly of hydrated ferric oxide on the surface of iron and steel articles, and other articles having a ferrous surface, which comprises providing said surface with a film of an aqueous solution of hydrochloric acid having a free acid content equivalent to about 0.2% to 3% hydrochloric acid, drying said film on said surface, and subjecting the filmed surface in dry form to the action of air having a relative humidity such as to cause the reaction to go to the ferric oxide stage, until the surface is uniformly covered with a red-brown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface.

8. The process for producing a uniform, smooth, bright, and strongly adherent coating consisting mainly of hydrated ferric oxide on the surface of iron and steel articles, and other articles having a ferrous surface, which comprises providing said surface with a film of an aqueous solution of chromic chloride having a free acid content equivalent to about 0.2% to 3% hydrochloric acid, drying said film on said surface, and subjecting the filmed surface in dry form to the action of air having a relative humidity such as to cause the reaction to go to the ferric oxide stage, until the surface is uniformly covered with a red-brown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface.

9. The process for producing a uniform, smooth, bright, and strongly adherent coating consisting mainly of hydrated ferric oxide on the surface of iron and steel articles, and other articles having a ferrous surface, which comprises providing said surface with a film of an aqueous solution containing a hygroscopic material and a chemical of the class consisting of acids and acid-reacting salts, said solution'having a free acid content equivalent.

a red-brown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface.

10. The process for producing a uniform, smooth, bright, and strongly adherent coating consisting mainly of hydrated ferric oxide on the surface of iron and steel articles, and other articles having a ferrous surface, which comprises providing said surface with a film of an aqueous solution containing an accelerator for the formation of hydrated ferric oxide selected from the group consisting of sulfites and thiosulfates and a chemical of the class consisting of acids and acid-reacting salts, said solution having a free acid content equivalent to about 0.2% to 3% hydrochloric acid, drying said film on said surface, and subjecting the filmed surface in dry form to the action of air having a relative humidity such as to cause the reaction to go to the ferric oxide stage, until the surface is uniformly covered with a red-brown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface.

11. In a process for effecting the plastic deformation of iron and steel articles, and other articles having a ferrous surface, the steps which comprise providing the surface of a said article with a film of an aqueous solution of a chemical of the class consisting of acids and acid-reacting salts, said solution having a free acid content equivalent to about 0.2% to 3% hydrochloric acid, drying said film on said surface, subjecting the filmed surface in dry form at atmospheric temperature to the action of air having a relative humidity such as to cause the reaction to go to the ferric oxide stage, until the surface is uniformly covered with a red-brown coating consisting mainly of hydrated ferric oxide which is firmly bonded to said surface, and plastically deforming the resulting coated article.

References Cited in the file of this patent UNITED STATES PATENTS 24,604 Pomeroy June 28, 1859 151,647 Allin June 7, 1874 1,333,586 Churchward Mar. 6, 1920 2,105,015 Singer Jan. 11, 1938 2,110,893 Sendzimir Mar. 15, 1938 2,203,063 Schueler June 4, 1940 2,350,491 Butler et al. June 6, 1944 2,366,208 Montgomery Ian. 2, 1945 OTHER REFERENCES Metal Progress, April 1945, pages 722 and 723. 

1. THE PROCESS FOR PRODUCING A UNIFORM, SMOOTH, BRIGHT AND STRONGLY ADHERENT COATING CONSISTING MAINLY OF HYDRATED FERRIC OXIDE ON THE SURFACE OF IRON AND STEEL ARTICLES, AND OTHER ARTICLES HAVING A FERROUS SURFACE, WHICH COMPRISES PROVIDING SAID SURFACE WITH A FILM OF AN AQUEOUS SOLUTION OF A CHEMICAL OF THE CLASS CONSISTING OF ACIDS AND ACID-REACTING SALTS, SAID SOLUTION HAVING A FREE ACID CONTENT EQUIVALENT TO ABOUT 0.2% TO 3% HYDROCHLORIC ACID, DRYING SAID FILM ON SAID SURFACE, AND SUBJECTING THE FILMED SURFACE IN DRY FORM TO THE ACTION OF AIR HAVING A RELATIVE HUMIDITY SUCH AS TO CAUSE THE REACTION TO GO TO THE FERRIC OXIDE STAGE, UNTIL THE SURFACE IS UNIFORMLY COVERED WITH A RED-BROWN COATING CONSISTING MAINLY OF HYDRATED FERRIC OXIDE WHICH IS FIRMLY BONDED TO SAID SURFACE. 